A twelve-pulse ATRU is a rectifier used to produce a DC output supply from a 3-phase AC power input. This type of rectifier is utilized, particularly in aircraft, because compared with the simplest possible 3-phase rectifier, a six-pulse rectifier, it gives a lower ripple on the DC output, and a lower level of harmonic currents in the three-phase input supply. In order to minimize the cost and weight of the phase-shifting transformer that is needed to realize the twelve-pulse scheme, it is made an auto-transformer.
It is important in such devices to be able to swiftly determine abnormalities such as a missing AC phase or a drop out of the input supply. If there is a missing phase only two of the three phases will be present at the rectifier and this can cause damage or malfunction of the equipment powered by the output of the ATRU. An input supply drop out can also be problematic in particular types of equipment powered by the output of the ATRU. For example in those which employ a sensorless square wave drive, it is vital that there is fast detection of an input supply drop out. This is needed because, in order to be able provide a ‘hot’ restart when the input power is reconnected, the bridge drive must be switched off within a very short time (typically 1 ms) of the input supply drop out, as the motor controller needs to keep track of the rotor position during the period the power is interrupted.
There are a number of existing proposals for detecting a missing phase. For example, U.S. Pat. No. 5,548,207 discloses a method in which the 3-phase supply being monitored is connected to the primary windings of an isolating transformer which steps down the voltages and supplies to them a rectifier. The ripple wave of the rectified output is analyzed to determine whether a missing phase is present. This technique requires a connection to the 3-phase supply and this may be problematic, particularly where access to or from the supply being monitored is awkward. For example, in the configurations proposed herein, the ATRU may be disposed within a fuel tank so that the fuel provides active cooling of the unit (thereby allowing weight reduction of the ATRU). The monitoring circuit will typically be external to the fuel tank and so in an arrangement of the type disclosed in U.S. Pat. No. 5,548,207, it would be necessary to provide additional sealed feed through connections to link the ATRU to the monitoring circuit.